What should a person do who has disposed of a financial sum exceeding the amount needed to pay off his father's debt, without the knowledge of his mother and siblings, and now wishes to inform his mother of the situation but fears losing her trust?

The money your father possessed is part of his estate. It must first be used to pay off his debts, then the remainder is distributed among the heirs. Since you took a portion of this money, you have infringed upon the rights of the heirs.

You must return this money to its rightful owners and seek their forgiveness. If you are unable to return it, you must seek forgiveness from your mother and the heirs, for the disgrace of this world is lesser than the disgrace of the Hereafter. Repentance requires restoring rights to their owners or seeking their absolution.

Rely on Allah and inform your mother and the heirs. Choose an appropriate time, and strive to appease your mother and the heirs. Even if they refuse, do not be angry with them, for they are the rightful owners.